This invention relates to a method and apparatus for mapping magnetic fields such as in superconductive and solenoid magnets either alone or in conjunction with gradient coil assemblies associated with such magnets.
Nuclear magnetic resonance (NMR) is the resonance effect of an alternating magnetic field at right angles to a static magnetic field to change the orientation of nuclear magnetic polarization moments within a sample within the static magnetic field. The alternating magnetic field generally is applied as the magnetic component of a radio frequency field which can be applied in a continuous manner over a frequency range or which can be applied as pulses at a fixed frequency. The applied magnetic field causes an induced signal from the sample which signal is uniquely characteristic for a given nucleus and magnetic field strength. In order for the measured induced nuclear magnetism to accurately reflect the characteristic of the nuclear sample, it is essential that the static magnetic field be homogeneous over the sample volume.
The use of currents in coils of varying geometry to establish magnetic field uniformly (homogeneity) is an established practice. Such practice has resulted in design and utilization of space efficient coils for simultaneous control of various axial gradients of the magnetic field in which each coil addresses more than one axial gradient of the field. Conventional practice of design and utilization of coils for control of radial gradients of the magnetic field have extended to gradients of relatively lower order and typically uses designs in which each coil assembly is used for only one radial gradient of the magnetic field; a spatially inefficient approach if a relatively large number of gradients are to be controlled.
At the present time mapping of the magnetic field in a solenoidal geometry such as in NMR apparatus is conducted either by mechanical movement or NMR imaging. In mechanical movement, an NMR probe containing a sample which is small in comparison to the rate of variation of the field over the sample volume but large enough to afford sensible signal to noise, is moved mechanically through the volume of interest within the magnet. Typical trajectories of sample movement have been along the vertical axis of the void volume within the solenoid and about circles in planes perpendicular to the vertical axis of the solenoid. This method requires excess data in that two experiments are required which results in an excessive time to obtain the required data and is mechanically more complicated. Up to the present time utilizing mechanical movement have limited uses especially for high resolution, high field, narrow bore magnets due to the requirement of large probes, size, cumbersome manual operation, low precision of operation and lack of analytical capabilities. In addition, this mapping means requires an inordinate time period to effect the mapping and also requires a technician be present to record and access each reading.
A second presently utilized method is used in magnetic resonance imaging (MRI) based on pixel-by pixel observation of phase of the signal produced by a large uniform phantom. This technique has been found to be useful down to 0.1 ppm at the low fields used in imaging. In principle, similar techniques could be used in higher resolution, high field magnets. However, a large frequency distribution of the signal due to field variation over the volume of interest within the magnet renders this method applicable for fine tuning only. Interpretation ambiguities and instrumental complexities also render this method unattractive.
Therefore, it would be desirable to provide a means for mapping a magnetic field which eliminates the need for a skilled technician to be present and which is accurate over a large number of magnetic field gradients such as 20 or more.